After President Trump’s Terrible Tuesday, Republican lawmakers need to stop pretending that there are any red lines that he won’t cross.

Congressional Republicans have been operating under a see-no-evil policy with President Trump: ignoring his lying, his subversions of democratic norms and his attacks on government institutions or, when that’s not possible, dismissing such outrages as empty bluster — as Trump being Trump.

..Also on Tuesday, a federal jury convicted Mr. Trump’s former campaign chairman, Paul Manafort, of bank and tax fraud. How did Mr. Trump react? More like a Mafia don than a guardian of the rule of law. While criticizing Mr. Cohen on Wednesday, the president tweeted that, by contrast, he had “such respect for a brave man” like Mr. Manafort, who “refused to ‘break’ … to get a ‘deal.’ ” The president, in other words, felt moved to praise a convicted felon for refusing to cooperate in the pursuit of justice.

.. And how did Republicans in Congress react? They didn’t, if they could avoid it. John Cornyn, the majority whip in the Senate, shrugged that he had “no idea about what the facts” of Mr. Cohen’s guilty plea were “other than the fact that none of it has anything to do with the Russia investigation.” The office of the House speaker, Paul Ryan, said it needed “more information.” Most members opted for silence.

.. When members of Mr. Trump’s party pooh-pooh his thuggish rantings and otherwise signal that they will overlook even his most dangerous behavior, they are inviting him to act out even more. Like a willful toddler, Mr. Trump lives to test limits.

.. Republican lawmakers need not attack Mr. Trump in order to stop enabling his worst impulses and begin distancing themselves from his corruption. They simply need to stop cowering. An obvious first step is for Congress to pass legislation protecting Robert Mueller’s Russia inquiry

.. The president has toyed with the idea of firing Mr. Muellerand his superior, Deputy Attorney General Rod Rosenstein, moves that would ignite a constitutional crisis. Lawmakers are deluding themselves to think that he won’t consider such radical acts again as his predicament grows more dire.

.. Much of the groundwork for a bill to protect the Russia investigation has already been laid, with a bipartisan plan having passed the Senate Judiciary Committee. Shamefully, Senator Mitch McConnell, the Republican majority leader, has refused to bring the bill up for a vote

.. insisting that it is unnecessary because of course the president would never fire Mr. Mueller.

.. Mr. Ryan has spouted similar assurances. Then again, Mr. Ryan also laughed off the idea that Mr. Trump would strip his political critics of their security clearances, so clearly Republican leaders are not the best barometers of this president’s thinking.

.. Speaking of Mr. Ryan, the speaker needs to shut down the attacks on Mr. Rosenstein by Mr. Trump’s lackeys in the arch-conservative Freedom Caucus.

.. Mark Meadows and Jim Jordan threatened to force an impeachment vote on Mr. Rosenstein, claiming that he was impeding Congress’s harassment — uh, “investigation” — of the Justice Department and the F.B.I. When that plan flopped, the men set their sights on holding Mr. Rosenstein in contempt of Congress — which doesn’t sound as dramatic, but would, if successful, provide Mr. Trump an excuse to oust Mr. Rosenstein and replace him with a lap dog.

.. Once upon a time, campaign finance violations made congressional Republicans very angry indeed. During Bill Clinton’s second term, there was quite an uproar over allegations that the Chinese government had attempted to influence the 1996 presidential race via illegal campaign contributions. (Does Vice President Al Gore’s visit to a certain Buddhist temple ring any bells?)

.. His efforts to hide the money trail suggest he knew his behavior wasn’t kosher. And while the initial payments to the women were made before Mr. Trump won the election, he didn’t begin compensating Mr. Cohen until February of 2017 — thus any conspiracy was carried straight into the Oval Office.

.. Every week seems to bring fresh evidence that Mr. Trump, his inner circle and his main backers do not consider themselves bound by such pedestrian concepts as truth, ethics or the law. The latest confirmation for that was the corruption indictment of Representative Duncan Hunter, Mr. Trump’s second campaign supporter in the House. The first, Representative Chris Collins, was indicted two weeks ago on insider-trading charges.

Congress, unfortunately, remains crouched and trembling in a dark corner, hoping this is all a bad dream. It’s not. Republican lawmakers need to buck up, remind themselves of their constitutional responsibilities and erect some basic guardrails to ensure that — in a fit of rage, panic or mere pique — this president does not wake up one morning and decide to drive American democracy off a cliff.